Which piece, when fitted into Figure (X), completes the square with no gap and no overlap? A piece may be turned round.

Answer & explanation
Correct answer: option 3
The correct answer is Option (3) →

The missing part of the 5 × 5 square in Figure (X) covers 5 small squares. The piece that fits must have exactly this shape and size. The piece in Option (3), turned through 180°, fills the gap exactly.

Option (1) — Incorrect. This piece has one small square more than the gap, so it cannot fit into it.
Option (2) — Incorrect. This piece covers as many small squares as the gap, but one of them is in a different place, so it does not fit the gap even when turned round.
Option (4) — Incorrect. This piece has one small square less than the gap, so a space would be left in the square.